Strain may be defined as deformation experienced by a body resulting from an application of force. Existing ways of sensing strain include, for example, computational approaches, individual transducers, and pressure sensors, such as pressure-sensitive films and pressure-sensitive mats. Existing sensors include metallic-based or micro-electro-mechanical system (MEMs)-based strain-measuring devices. These sensors are generally fixed sizes and fixed shapes. Also, these typical sensors are rigid and flat, and hence, usually cannot be used for measuring strains on irregular and curved surfaces. Further, existing sensors are relatively expensive and neither flexible nor machinable. For example, MEMS type semiconductor and fiber-optic strain sensors can achieve high sensitivities, but have high manufacturing costs and require costly data acquisition systems.
Commercially available constantan or nickel-chromium-alloy-based strain gages offer wide static, dynamic, and temperature ranges. However, these gages also lack versatility and flexibility, as these gages can usually only measure strains at specific locations to which the gauges are bonded and along a directional grid. In addition, these gages typically exhibit a relatively low and narrow range of gauge factor, such as from 2.0 to 3.2.
The gauge factor of a strain sensor is defined as the relative change in the electrical resistance of the sensor for an applied mechanical strain. R0 may be the resistance of the sensor under a no-strain condition, and the resistance may increase to Rε under the application of a strain ε. Ignoring any temperature effects, the gauge factor, G, of a strain sensor may be given by the relationship:
  G  =                              (                                    R              ɛ                        -                          R              0                                )                /                  R          0                    ɛ        =                  Δ        ⁢                                  ⁢                  R          /                      R            0                              ɛ      Gauge factor serves as an index of sensitivity of a sensor to mechanical strain. A higher gauge factor indicates more strain sensitivity. For example, the larger the gauge factor is, the smaller the strains that may be detectable by a sensor.